Yes, I explicitly set it as either the web map extent or the current map extent in the builder, but it won't zoom to that extent on other machines. It may not be the aspect ratio. It may be the height. It's like it doesn't accurately understand that the extent it's supposed to display is not completely visible.
Here's what I mean. The app below was just refreshed. It is on my machine where I configured the web app, but the browser window was made smaller. The map features are cut off.
When fully maximized on my machine, it looks like this. I understand the browser window may change size, but I would think the initial app extent should always be visible on load.